The present invention relates generally to catalysts comprising ruthenium oxide and to processes for catalyzing the oxidation and conversion of sulfur dioxide (SO2) to sulfur trioxide (SO3) using such catalysts. More particularly, SO2 at low concentrations in process gas streams can be effectively oxidized to SO3 at relatively low temperatures using the ruthenium oxide catalysts of the present invention. For example, the catalysts comprising ruthenium oxide are particularly useful for conversion of SO2 to SO3 in the final contact stage of a multi-stage catalytic converter used in sulfuric acid manufacture.
The conventional contact process for the manufacture of sulfuric acid comprises catalytic gas phase oxidation of SO2 to SO3 in one or more catalytic oxidation stages of a converter to produce a conversion gas comprising SO3, and absorbing the SO3 in aqueous sulfuric acid to form additional sulfuric acid product. The catalytic oxidation of SO2 to SO3 proceeds at useful rates over solid particulate catalysts typically containing alkali-vanadium or platinum-containing active phases. SO2 gas concentrations at the inlet to the first catalytic stage of the converter usually range from about 4% to about 15%. With adiabatic operation of each stage of the converter, three or four catalytic stages (or passes) are generally required to achieve overall SO2 conversions in excess of 99.7% and satisfy absorber tail gas emission standards. External heat exchangers typically precede each catalyst pass following the first pass in order to cool the gas stream to the desired inlet temperature, with the fourth stage typically operating at from about 360° C. to about 415° C. Conversions of 99.7% of the first stage inlet SO2 concentration are suitably obtained through a four stage double absorption design in which SO3 is removed from the gas stream through a sulfuric acid irrigated absorption tower that follows the second catalytic stage (2:2 interpass absorption (IPA) design) or the third catalytic stage (3:1 IPA design) of the converter. SO2 conversion of about 94% to about 95% is generally achieved in the first three stages, leaving the remainder to be converted in the fourth, or final, catalytic stage of the converter prior to passage through a final absorption tower for recovery of additional sulfuric acid product.
Prior art processes, such as described in U.S. Pat. No. 5,264,200 to Felthouse et al., effectively achieve a high total SO2 conversion and acceptable SO2 emission levels in the absorber tail gas by contacting the SO2-containing gas with a monolithic catalyst having a platinum or alkali-vanadium-containing active phase in a series of preliminary catalytic stages prior to interpass absorption followed by a further pass through a final catalytic stage containing a particulate vanadium catalyst containing cesium (i.e., a Cs—V catalyst). By the use of a particulate Cs—V catalyst, the final stage reaction can proceed to thermodynamic equilibrium with a low inlet gas temperature range of from about 360° C. to about 415° C., a temperature range that favors a high conversion of SO2 to SO3.
Tomas Jirsak et al. in “Chemistry of SO2 on Ru(001): formation of SO3 and SO4,” Surface Science 418 pp. 8-21 (1998) describe the exposure of ruthenium (001) crystal to SO2 and oxygen resulting in disassociation of SO2 or decomposition or disproportionation that leads to the formation of SO3 and SO4.
In an effort to achieve economies of scale, contact sulfuric acid plants often are built with capacities of 1500 to 2500 metric tons per day (as 100% H2SO4). That rate of production requires relatively large diameter (e.g., 5 to 15 meter) catalytic converter vessels containing catalyst loadings on the order of from about 30 to about 50 liters per metric ton (as 100% H2SO4), or more, per stage. Increased catalytic efficiency would enable the use of lower catalyst loadings. Desirably, additional SO2 conversion efficiency and lower process emissions could be attained through the use of a final stage catalyst having improved low temperature activity as compared to known SO2 oxidation catalysts. There is a need, therefore, for an SO2 oxidation catalyst that is stable and possesses high activity thereby enabling reduced catalyst loading requirements, higher gas velocities and associated reduced capital costs.